Sounds to me like you may need to set the formatting on the first page (Select File tab) of the Import wizard.
Under the section 'Samples of Date (1, August), Time and Number for Selected Locale', ensure that WFM has the right formatting selected by ticking the box to the right of the date field and change it to match your source data.
If this has already been set correctly, review your source data and ensure all values in the Date field are in fact formatted as you expect. Sometimes when copying and pasting data to Excel, the formatting may only be filled down half way and changes back to 'General' formatting at some point in the list. A quick way to validate this may be to proceed to the 'Select Data Columns' tab and see if the Last Row values look right.
You may also need to do the same for the Interval field.